For the first time in modern history, Shiite fighters are crossing borders to wage jihad, much as Sunnis traveled to Afghanistan to fight the Soviets and to Iraq to fight Americans. These days, the draw is Syria. The cause is not a foreign invader but a rival sect. And some say, it's about fulfilling a thousand-year-old prophecy, as NPR's Kelly McEvers reports.

KELLY MCEVERS, BYLINE: Abu Ther is a short guy but a big guy. He keeps his beard cropped close, the Shiite way. His smile is almost a smirk. He's ready to show off.

Abu Ther is from Iraq. We're sitting with him in Baghdad. The videos on his phone are from Syria. In this one, Abu Ther and about a dozen other guys are ripping the plastic off some brand new guns and rocket-propelled grenades; weapons he says they later used against Syrian rebels.

That's him? And he's loading a mortar tube, right?

UNIDENTIFIED WOMAN: (Foreign language spoken)

ABU THER: (Foreign language spoken)

MCEVERS: All right, yep. He's loading it into the tube now, leans over.

(SOUNDBITE OF EXPLOSION)

MCEVERS: For the past eight or nine months, Abu Ther says the unit he commands has been going to fight against rebels in Syria.

(SOUNDBITE OF YELLING AND GUNSHOTS)

MCEVERS: To Abu Ther, these rebels are a threat to his very existence. Abu Ther is a Shiite. The rebels are, by and large, Sunni.

(SOUNDBITE OF MEN SINGING)

MCEVERS: In this video, Shiite fighters dance and sing and eventually fall to the ground in prayer. They claim they've just regained control of an airport in Syria. Their accents are Lebanese and Iraqi.

(SOUNDBITE OF MEN SINGING)

MCEVERS: Syria's war didn't start as a sectarian one. It started as an uprising of the people against their government. But the majority of those people are Sunni. The government is Alawite, which has roots in Shiite Islam.

THER: (Foreign language spoken)

MCEVERS: Abu Ther says he's not fighting in Syria to support the Syrian president, Bashar al-Assad.

THER: (Foreign language spoken)

MCEVERS: He's fighting to stop what he sees as an onslaught of Sunnis led by jihadi militants. Abu Ther says there are hundreds of Iraqi Shiites now fighting in Syria. They're joined by thousands of Lebanese Shiites from the militant group, Hezbollah. Abu Ther says the fighters are trained by Hezbollah leaders and the operation is overseen by Iran.

But perhaps the most striking thing Abu Ther tells us is that this war has been pre-ordained by one of the most revered figures in Shiite Islam, the Imam Mahdi, who lived in the 9th century. Abu Ther says the imam predicted he would reappear on Earth after an army from Syria, led by a man named Soufyani, attacked the Shiite shrines of Iraq.

THER: (Through Translator) We stick to those teachings and according to the teachings that we have to fight this Soufyani army. And this is what we are doing. But instead of fighting them inside Iraq, we want - we are fighting them inside their own territories.

Vali Nasr heads the Johns Hopkins School for Advanced International Studies and has written widely about Shiites in the Middle East. He says the Imam Mahdi's prophecies been employed before during Iran's Islamic Revolution of 1979, in the Iraq War when Shiites fought U.S. forces. Nasr says it's no surprise the story has come up again. The people on the ground might believe it but the leaders at the top know differently.

VALI NASR: This is very much a strategic thinking. It's not religious thinking. This is a game of power. This is really a game of power about who controls Iraq, Lebanon and Syria. And for Iran, this is a regional power play.

NASR: Iran has already proven something very important, which is that it's the only country in the region with the capability to carry out a large-scale military operation in another country not on its border. That's something that gives them an aura of power.

MCEVERS: That their main rivals, the U.S. and the Sunni-led countries in the region currently can't match, he says.

(SOUNDBITE OF MUSIC)

MCEVERS: It's a projection of power we're starting to see all over the region, from Facebook to ringtones, to flags in poor Shiite neighborhoods. All of them talk of the coming regional war between the righteous and the evil, between one sect and another.